Gun control activists acknowledge they’re going to have to play defense in Washington against a government dominated by Republicans, but they insisted Wednesday that the politics of the issue are moving in their direction nationally.

Despite going all-in during last year’s presidential election and seeing their candidate lose to the fervidly pro-gun President Trump, the activists rallying at the Capitol said they believe they can stop the National Rifle Association’s chief goal of national reciprocity for concealed-carry permits.

“We’ve clearly got Congress thinking twice — even starting to get a little scared of us,” said Dan Gross, president of the Brady Campaign to Prevent Gun Violence.

Activists touted a surge of phone calls made by their side to members of Congress as evidence of a growing movement. And lawmakers pointed to specific races last year where guns were an issue.

Sen. Chris Murphy, Connecticut Democrat, pointed to the change in New Hampshire, where six years ago he said the U.S. Senate race was a battle for the NRA’s endorsement.

“Because of the movement that you have all built, just six years later two candidates for Senate in New Hampshire, one of them being the incumbent, were trying to outflank each other to the left to show that they were better on gun violence,” Mr. Murphy said. “The politics have changed. The movement is bigger and stronger now than ever.”

But Second Amendment backers shrugged off those claims.

“There’s always going to be a few cases here and beyond where maybe somebody was anti-gun that gets elected,” said Philip Van Cleave, president of the Virginia Citizens Defense League. “Well, it could be for other reasons.”

Mr. Van Cleave said guns simply don’t present the single-issue motivator for gun control activists the way the issue still does for gun-rights supporters.

“On our side, it’s just the opposite. Our rights are under attack,” he said. “We’re motivated in ways that they’re not motivated and will never be motivated.”

Lawmakers on Wednesday did acknowledge that at least at the federal level, activists are going to be playing defense in the near future, and Mr. Murphy said the NRA expects something from the group’s endorsement of President Trump.

“They are going to be on the offense in this building,” Mr. Murphy said. “We are going to have to play some defense here to stop some pretty terrible pieces of legislation from becoming law, whether it be the ability to carry a concealed weapon anywhere in the United States, or whether it be the legalization of deadly silencers.”

The activists were joined by former Rep. Gabrielle Giffords, who was gravely wounded by a deranged shooter at a constituent event in Arizona in January 2011.

Mrs. Giffords’ group, Americans for Responsible Solutions, is teaming up with the Brady Campaign to host an all-day strategy session on Thursday in Washington to figure out how to slow down groups like the NRA.

Mr. Van Cleave acknowledged the potential for complacency among gun-rights activists satisfied with last year’s wins, but said he’ll certainly be reminding his members that they shouldn’t squander the opportunity they have now.

“Now is the time when these people should be awake because now is a chance that we haven’t had in a long time to roll back gun control — to move gun rights forward,” he said.